Current control for output device biasing stage

ABSTRACT

Circuits and methods to control current through a device biasing an output device in case the supply voltage is not higher than the output voltage are disclosed. The circuits and methods are applicable to e.g. LDOs, amplifiers, or buffers. A control loop detects if the supply voltage is not higher than the output voltage and regulates the drain-source voltage of the biasing device. The disclosure reduces power consumption in a driver stage in case the supply voltage is not higher than the output voltage.

This is a divisional application of U.S. patent application Ser. No.13/790,401, filed on Mar. 8, 2013, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

BACKGROUND

1. Technical Field

The present document relates to low drop-out (LDO) voltage regulators,all type of converters, buffers, and amplifiers. In particular, thepresent document relates to methods and systems for maintaining currentconsumption in case the supply voltage equals or comes close to therequired output voltage.

2. Background

Prior art implementations of circuits as e.g. low-dropout (LDO) voltageregulators, buffers, or amplifiers are losing control over powerconsumption in case the supply voltage equals or comes close to theoutput voltage.

This leads to following disadvantages of prior art:

-   -   Unnecessary power consumption.    -   Adverse load transient behavior under dropout conditions.    -   Bypass mode can only be implemented with high power consumption    -   Drop in voltage of the voltage (brown out).    -   No monitoring of dropout condition.

It is a challenge for engineers to design circuits as e.g. LDOs,buffers, or amplifiers enabled to control power consumption while thesupply voltage is coming close or equal to required output voltage.

SUMMARY

A principal object of the present disclosure is maintain powerconsumption control of LDOs, buffers or amplifiers while the supplyvoltage is lower or equal to required output voltage.

A further object of the disclosure is to avoid an output voltage drop(brown out) condition of the circuit as e.g. an LDO.

A further object of the disclosure is to reduce current consumptionunder no-load in a driver stage of a battery supplied device if thebattery voltage becomes close or equal to required output voltage.

A further object of the disclosure is to improve load transient behaviorunder dropout condition.

A further object of the disclosure is to implement a bypass mode withreduced power consumption.

A further object of the disclosure is to implement an automatic bypassmode.

A further object of the disclosure is to monitor dropout conditions.

A further object of the disclosure is to add a control loop to controlthe drain source voltage across the transistor providing a biasingvoltage for an output device of the circuit without impacting the ACstability of the system.

A further object of the disclosure is to deploy a control loopconsisting of an amplifier and a comparator.

In accordance with the objects of this disclosure a method of currentcontrol of a biasing stage of electronic systems comprising a biasingdevice, biasing an output device in case a supply voltage is equal orclose to desired output voltage, has been achieved. The method disclosedcomprises the following steps: (1) providing an electronic system havinga supply voltage, an output device, and a correspondent biasing device,(2) detecting by a controller if supply voltage is not higher than anoutput voltage reduced by a threshold dropout voltage, and (3)maintaining a drain-source voltage across the biasing device to be thesame as a drain-source voltage across the output device in case thesupply voltage is not higher than output voltage reduced by a thresholddropout voltage by the controller.

In accordance with the objects of this disclosure a circuit to controlcurrent through a biasing stage of electronic systems comprising anbiasing device, biasing an output device, in case a supply voltage isless or equal to desired output voltage, has been achieved. The currentcontrol circuit comprises: a controller configured to detect if thesupply voltage is not higher than or close to the output voltage by avoltage difference and, if this is the case, controls the biasingdevice.

In accordance with the objects of this disclosure a circuit to controlcurrent through a biasing stage of electronic systems comprising anbiasing device, biasing an output device, in case a supply voltage isless or equal to desired output voltage, has been achieved. The circuitfirstly comprises: the output device having a source connected to supplyvoltage, a drain connected to an output port and a gate connected to agate of the biasing device and to a second terminal of a resistor, saidoutput port, and the biasing device having a source connected to supplyvoltage and a drain connected to a source of a control transistor outputport. Furthermore the circuit comprises said control transistor having agate connected to an output of a controller and a drain connected to afirst terminal of an input device of the biasing stage, said inputdevice of the biasing stage; and said controller having inputs and anoutput, wherein a first input is the supply voltage, a second input is avoltage representing the output voltage of the electronic device, andthe output is connected to the gate of the control transistor, whereinthe controller detects if the supply voltage is not higher than theoutput voltage reduced by a threshold dropout voltage and, if this isthe case, controls the drain-source voltage of the biasing device viathe control transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIG. 1 shows basic elements of a circuit to control the drain-sourcevoltage across a transistor biasing an output device using a controlloop when supply voltage is less or equal or slightly higher, but lessthan dropout voltage, than a desired output voltage.

FIG. 2 illustrates a detailed schematic of the control loop.

FIG. 3 illustrates a flowchart of a method of current control of abiasing stage in case a supply voltage is less or equal to desiredoutput voltage.

FIG. 4 shows basic elements of an implemented circuit to control thedrain-source voltage across a transistor biasing an output device usinga control loop.

FIG. 5 shows basic elements of the circuit disclosed implemented as partof a LDO.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Methods and circuits to achieve current consumption control by a biasingstage for an output device of electronic systems as e.g. an LDO, abuffer, or an amplifier.

FIG. 4 shows an implementation of a circuit to control the drain-sourcevoltage across a transistor biasing an output device using a controlloop. FIG. 4 depicts a port for the output voltage 7, an output device1, a biasing device 2, providing bias current for the output device 1.

Disadvantages of the circuit illustrated in FIG. 4 are

-   -   High undesired current consumption under no-load condition in        the diode-driver stage in case of supply voltage, e.g. battery        voltage, becomes less than or equal to a desired output voltage;        and    -   Problematic load transient behavior under dropout conditions.

FIG. 1 illustrates the basic elements of a circuit to control a devicebiasing an output device using a control loop when supply voltage isclose to or equal to a desired output voltage while solving thedisadvantages mentioned above.

FIG. 1 shows a supply voltage, a port for the output voltage 7, anoutput device 1, a biasing device 2, and a control loop 3 controllingthe drain-source voltage of the biasing device 2 via control transistor8. Any type of controller could be used for the control loop 3.

In a preferred embodiment of the disclosure both biasing device 2 andoutput device 1 are p-channel MOSFETs enhancement type. Other transistortypes would be also possible for the biasing and output devices. Thegates of the biasing device 2 and of the output device 1 are directlyconnected to each other in a current mirror configuration.

The control loop 3 receives as input the supply voltage and the outputvoltage 7. In case the supply voltage is less or equal to the requiredoutput voltage 7, the output of the control loop 3 is connected to thegate of control transistor 8 in order to control the biasing device in away that the drain-source voltage of the biasing device 2 keeps the samevalue as the drain-source voltage of the output device 7. In a preferredembodiment of the disclosure the control device 8 is e.g. a p-channelMOSFET enhancement type. Under normal operating conditions, i.e. thesupply voltage is higher than the output voltage 7, the “Control” signalis pulled to ground and the device 8 acts as a completely ON switch. Thebiasing device and output device may or may-not be matched. But the sizeof biasing device is smaller than the output device.

It should be noted that the current consumption in the driver stageunder zero load condition is significantly reduced due to the control ofthe control loop 3 and the current mirror configuration of the biasingdevice 2 and output device 1.

Furthermore the circuit may comprise an output voltage divider 5,comprising resistive means R1 and R2, an optional resistor between thecontrol transistor 8 and the input transistor 6. The optional resistormay be used as another way of limiting the current in the biasing stage.

FIG. 2 illustrates a detailed schematic of the control loop 3 comprisingan amplifier and a comparator. Its inputs comprise the supply voltageand the output voltage 7. Its outputs comprise a control output to thegate of control transistor 8 and an output to a dropout monitor.

The dropout monitor is a digital signal or a digital flag forcommunication to a host or controller or external world. It provides theinformation that supply voltage is close to the regulated outputvoltage. It can be used for automatic bypass mode implementation. Thevoltage difference between the supply and output voltage when this flagwould toggle is programmable.

In a preferred embodiment of the invention the control loop of FIG. 2comprises three MOS FET transistors 20-22 and three current sources23-25, wherein each of the drains of the transistors 20-22 is connectedto one of the current sources 23-25.

The combination of transistors 21 and 22 with current sources 24 and 25forms a common gate amplifier. This common gate amplifier generates the“Control” signal to control the gate of device 8 shown in FIG. 1.Transistor 22 is a diode connected PMOS transistor connected to thesupply voltage and to current source 25 providing the bias current fortransistor 22 to generate a voltage which is one threshold voltage belowthe supply (“gate control”). This voltage controls the gates oftransistors 20 and 21.

During normal operation the output voltage is lower than supply voltage,this leads to “Control” and “Dropout Monitor” being pulled to negativerail.

The combination of transistors 20 and 22 with current sources 23 and 25make a common gate comparator. This combination generates the “DropoutMonitor” signal that acts as a flag to a controller or host or externalworld. This can also be used for implementing an automatic bypass mode.Usually the size of transistor 20 (“Y”) is greater than the size oftransistors 21 or 22 (“X”). Alternatively the same functionality can beimplemented by different current in current sources 23 and 25.

When the difference between the supply voltage and regulated outputvoltage is higher than the specified dropout voltage, i.e. “normal”operating condition, both “Control” and “Dropout Monitor” voltage levelsare pulled to the most negative rail. The control device 8, shown inFIG. 1, acts then as a closed switch under this condition.

FIG. 5 shows basic elements of the circuit disclosed implemented as partof a LDO. FIG. 5 shows a typical LDO resistive voltage divider R1/R2providing feedback from the output voltage 7 to a differential erroramplifier 50, comparing a reference voltage Vref with the voltage of afeed-back point of the voltage divider R1/R2.

Transistor MN1 and the current source form an internal stage of the LDOused as an example. In any other circuit that uses the biasing stage asshown in FIG. 4 (with optional resistor 9) transistor MN1 and currentsource are optional.

A miller capacitor 51 is deployed for frequency compensation. The Millercapacitor 51 is used for compensation in the LDO taken as example. Butit may be optional in any other circuit using the biasing stage of FIG.4 for biasing with optional resistor 9.

The input device 6 of the biasing stage receives input via thetransistor MN1 52. As disclosed above the control loop 3 of the biasingstage controls the drain source voltage across the biasing device 2providing the biasing voltage of the output device 1. The control loop 3maintains the drain-source voltage across the biasing device 2 the sameas the drain source voltage across the output device 1 when the supplyvoltage driving the output device 1 is lower or equal or very close tothe output voltage 7 required.

FIG. 3 illustrates a flowchart of a method of current control of abiasing stage of electronic systems, comprising a biasing device biasingan output device, in case a supply voltage is less or equal to desiredoutput voltage. As already mentioned above, the circuit and the methoddisclosed are applicable to electronic systems having an outputtransistor and a related biasing device as e.g. an LDO, a buffer, or anamplifier.

Step 30 of the method of FIG. 3 illustrates the provision of anelectronic system having a supply voltage, an output device, and acorrespondent biasing device. Such a device could be e.g. an LDO, anamplifier, or a buffer. Step 31 depicts detecting if supply voltage isnot higher than output voltage by a controller. Step 32 showsmaintaining a drain-source voltage across the biasing device to be thesame as a drain-source voltage across the output device in case thesupply voltage is not higher than output voltage by the controller.

While the disclosure has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the disclosure.

What is claimed is:
 1. A method of current control of a biasing stage ofelectronic systems comprising a biasing device, biasing an output devicein case a supply voltage is less than or equal to a desired analogoutput voltage, comprising the following steps: (1) providing anelectronic system having a supply voltage, a MOSFET output device, and acorrespondent MOSFET biasing device; (2) detecting by a controller ifthe supply voltage is not higher than the analog output voltage reducedby a threshold dropout voltage; and (3) maintaining a drain-sourcevoltage across the MOSFET biasing device to be the same as adrain-source voltage across the MOSFET output device in case the supplyvoltage is not higher than the analog output voltage reduced by thethreshold dropout voltage by the controller wherein a gate of the MOSFETbiasing device connected to a gate of the MOSFET output device in acurrent mirror configuration.
 2. The method of claim 1 wherein saidelectronic system is a low-dropout (LDO) regulator.
 3. The method ofclaim 1 wherein said electronic system is an amplifier.
 4. The method ofclaim 1 wherein said electronic system is a buffer.
 5. The method ofclaim 1 wherein said controller is a circuit comprising a comparator andan amplifier.
 6. The method of claim 5 wherein the drain-source voltageof the MOSFET biasing device is regulated by said controller viacontrolling a gate voltage of a control transistor, which is deployed inseries with the MOSFET biasing device.
 7. The method of claim 5 whereinsaid controller provides a digital flag signal providing informationthat the supply voltage is close to the regulated input by aprogrammable voltage difference.
 8. The method of claim 7 wherein saiddigital flag can be used for an automatic bypass mode implementation.